Both commercial and military markets currently exist for imaging devices such as hand-held, helmet-mounted, and rifle-mounted scopes that integrate night vision or thermal imaging capabilities. There are several shortcomings to the existing technology. Night vision scopes use image intensifier tubes (IITs), which amplify low levels of visible light and produce “green” night vision images. IITs have the benefit of a relatively simple system with an integrated device that both detects and displays the image. However these scopes are susceptible to being blinded by ambient visible light (“blooming”). When background illumination is not sufficient, a near infrared illuminator (usually a laser) may be used to provide more reflected light. But these illuminators are easily seen by other cameras, even silicon-based cameras that are not designed for night vision. Most importantly, the IIT-based imager must be completely changed out for a traditional scope for daytime operation. Alternatively, thermal imaging scopes use a thermal camera to acquire a digital image that it then displays on an electronic display. A significant benefit is that this device is simply an add-on to a standard rifle scope, where the displayed thermal image is placed in line with the existing scope. This allows the night vision function to be quickly added or removed as conditions require. However, the camera/process/display architecture requires separate imaging, processing, and display systems, which consumes additional power. The thermal imaging components are generally quite expensive, and reflected light images are often easier to use to identify objects and therefore preferred over thermal images.
A photodetector may form the basis of an imaging device such as, for example, a digital camera capable of producing still photographs and/or video streams from an observed scene. The imaging device in such applications typically includes a light-sensitive focal plane array (FPA) composed of many photodetectors and coupled to imaging electronics (e.g., read-out chips). The photodetector of a typical digital camera is based on silicon technology. Silicon digital cameras have offered outstanding performance at low cost by leveraging Moore's Law of silicon technology improvement. The use of silicon alone as the light-absorbing material in such cameras, however, limits the efficient operation of these cameras in the infrared spectrum. Silicon is therefore not useful in the portion of the electromagnetic spectrum known as the short-wavelength infrared (SWIR), which spans wavelengths from ˜1.0 to 2.5 μm. The SWIR band is of interest for night vision applications where imaging using night glow and reflected light offers advantages over the longer thermal infrared wavelengths. Moreover, SWIR imaging is useful, for example, in military surveillance and commercial security surveillance applications and is considered to have technological advantages over MWIR and LWIR imaging, but thus far has been limited to use in high-performance military applications due to the high costs associated with traditional design and fabrication approaches. Additionally, while detector arrays exhibiting good sensitivity to incident IR radiation have been developed based on a variety of crystalline semiconductors, such arrays conventionally have been required to be fabricated separately from the read-out chips or other electronics utilized in the imaging device. Conventionally, after separately fabricating a detector and a read-out chip, these two components are subsequently bonded together by means of alignment tools and indium solder bumps, or other flip-chip or hybridization techniques to form an FPA. This also adds to fabrication complexity and expense.
Conventionally, photodetector devices and other optoelectronic devices have utilized bulk and thin-film inorganic semiconductor materials to provide p-n junctions for separating electrons and holes in response to absorption of photons. In particular, electronic junctions are typically formed by various combinations of intrinsic, p-type doped and n-type doped silicon. The fabrication techniques for such inorganic semiconductors are well-known as they are derived from many years of experience and expertise in microelectronics. Detectors composed of silicon-based p-n junctions are relatively inexpensive when the devices are small, but costs scale approximately with detector area. Moreover, the bandgap of Si limits the range of IR sensitivity to ˜1.1 μm. Group III-V materials such as indium-gallium-arsenide (InxGayAs, x+y=1, 0≦x≦1, 0≦y≦1), germanium (Ge) and silicon-germanium (SiGe), have been utilized to extend detection further into the IR but suffer from more expensive and complicated fabrication issues.
More recently, quantum dots (QDs), or nanocrystals, have been investigated for use in optoelectronic devices because various species exhibit IR sensitivity and their optoelectronic properties (e.g., band gaps) are tunable by controlling their size. Moreover, QD layers may be formed by relatively low-cost solution-based processes and deposited by low-cost processes such as spin-coating, printing, etc., as described in above-referenced U.S. Patent Pub. Nos. 2012/0241723 and 2012/0223291. Thus far, however, optoelectronic devices incorporating QDs have typically exhibited less than optimal performance due to factors such as low carrier mobility and short diffusion length.
To leverage the low cost and SWIR spectral sensitivity of QD detectors for low light level imaging a straightforward method of amplifying a QD detector signal is needed. Furthermore tying the output of this amplification stage to a device that emits light in the visible spectral region would allow the creation of low cost imaging system that is simple and straightforward like that of an IIT. Unlike an IIT, however, such a device would be sensitive to SWIR light, not suffer from blooming, could be quickly added or removed from an existing rifle scope, and may be suitable for day or night use. Furthermore it would offer the detection capabilities of indium gallium arsenide detectors without InGaAs's high costs and without the added power requirements that come with the circuitry used in a digital camera.
The reference Jun Chen, Dayan Ban, Michael G. Helander, Zheng-Hong Lu, and Philip Poole, “Near-Infrared Inorganic/Organic Optical Upconverter with an External Power Efficiency of >100%,” Advanced Materials, 2010, 22, 4900-4904, is incorporated by reference herein. The reference discloses an upconversion device with gain, composed of an InxGa1-xAs phototransistor and an OLED visible emitter. The phototransistor provides both detection and gain in a single two-terminal component. This has the advantage of simplicity in architecture, as the two terminals may be connected on the bottom and top of the layer stack, respectively. However it limits gain control and design flexibility between the gain unit and the detector unit.
The reference Franky So, Do Young Kim, Jae Woong Lee, Bhabendra K. Pradhan, “A method and apparatus for detecting infrared radiation with gain,” WO2013/003850 A2, is incorporated by reference herein. The reference discloses an upconversion device with gain. The detector in So et al. is a different type than in Chen et al., but is similar in using a two-terminal device that acts as both the detector and gain element, and is connected in series with an LED.
The reference Ken-ichi Nakayama, Shin-ya Fujimoto, and Masaaki Yokoyama, “Improvement in the on/off ratio of a vertical-type metal-base organic transistor by heat treatment in air,” Organic Electronics, 2009, 10, 543-546, is incorporated by reference herein. The reference discloses a method for fabricating a vertical channel thin film transistor (TFT) at low temperature in a thin film stack, known as a metal base organic transistor (MBOT). The gain and on/off ratio of this device substantially exceed values that have been demonstrated previously for this device type. The demonstrated device has a collector, a first TFT active region, a base, a second TFT active region, and a collector, with the collector, base, and emitter electrodes arranged in a vertical stack.
Therefore, there is a need for low-cost, integrated SWIR-to-Vis upconversion devices that detect visible to SWIR images and upconvert them to visible images in real time.